The present invention relates generally to a progressive ultrasonic spot welding system and, more particularly, to such a system wherein forging pressure and ultrasonic welding power levels are closely monitored and coordinated during the welding cycle so that the weld spot is started as a small point in comparison to its final size and is then progressively expanded to full size. Forging pressure and welding power are synchronously ramped and increased in conjunction with one another. The system further provides for an automatic sequential operation of a series of welding functions which, heretofore, had existed as independent manual operations thus providing a faster and more repeatable weld procedure with reduced operator burden. The system achieves positive pressure and power control by providing a single synchronization signal that programs pressure and power application for the weld cycle.
Ultrasonic vibratory spot welding processes for joining together two or more similar or dissimilar materials have been used for a number of years. Until recently, however, such methods were limited to use on thermoplastics, non-woven fabrics and metals where weld strength and integrity were not particularly important. This limitation was due, in large measure, to the problems associated with the ultrasonic welding methods employed, most of which were in prototype stages. In those instances when weld strength and weld integrity were important, i.e., when joining together structural aircraft panels and the like, resistance spot welding techniques were used.
Ultrasonic spot welding procedures have recently demonstrated strong potential for improved sheet metal assembly at reduced cost when compared with resistance spot welding and adhesive bonding techniques. Early studies have indicated that welds effected using prototype ultrasonic welding equipment such as, for example, a Sonobond M-8000 ultrasonic spot welder, were superior to welds produced using conventional resistance spot welding procedures. These early trials indicated that for virtually any material combination, an ultrasonically produced spot weld has an ultimate yield strength of more than 2.5 times that of a weld produced using resistance spot welding equipment. Further tests have indicated that ultrasonically produced spot welding can be accomplished with a 75% time and cost savings over conventional adhesive bonding techniques. Until now, however, ultrasonic spot welding for large structural metal parts was not possible in a production environment because of the numerous problems associated with the procedures.
Ultrasonic vibratory welding is a metallurgical joining technique which utilizes high frequency vibrations to disrupt the surface films and oxides and which, therefore, promotes interatomic diffusion and plastic flow between the surfaces in contact without any melting of the materials. Briefly stated, the ultrasonic welding process consists of clamping or otherwise securing together the workpieces under moderate pressure between the welding tip and a support anvil and then introducing high frequency vibratory energy into the pieces for a relatively short period of time, i.e., from a fraction of a second to a number of seconds. In many instances, the pieces to be welded are also adhesively bonded together by the insertion of an adhesive bonding agent between the juxtaposed pieces before welding which results in a high strength joint with superior static and fatigue properties.
One example of an ultrasonic spot welder particularly adapted for use on structural metal workpieces is the Sonobond Model M-8000 ultrasonic spot welder marketed by Sonobond Corporation of West Chester, PA. This welder includes a transistorized, solid state frequency converter which raises standard 60 Hz electrical line frequency to 15-40 kHz and then amplifies the output. The high frequency electrical power travels through a lightweight cable to a transducer in the welding head where it is converted to vibratory power at the same frequency. The vibratory power is, thereupon, transmitted through an acoustic coupling system to the welding tip and then through the tip into and through the workpieces, with the vibratory energy effecting the weld.
The Sonobond M-8000 ultrasonic spot welder includes a wedge-reed transducer coupling system which transmits lateral vibrations of a perpendicular reed member attached to it so that the welding tip at the lower end of the reed executes shear vibrations on the surface of the workpieces. The transducer includes piezoelectric ceramic elements encased in a tension shell assembly and operates at a nominal frequency of 15 kHz. A solid state frequency converter with a transistorized hybrid junction amplifier powers the welder. The converter operates at a nominal frequency of 15 kHz with a power output variable up to about 4000 RMS RF watts. The welder may be tuned to a precise operating frequency. The frequency converter includes a wide-band RF power measuring circuit which samples output power and detects forward power and load power based on the principle of bi-directional coupling in a transmission line. The signal is processed electronically to provide true RMS values which are selectively displayed on a light emitting diode (LED) panel meter as either the forward or load power. Forward power is the output of the frequency converter delivered to the transducer in the welding head while load power is the transducer drive power acoustically absorbed by the anvil. The difference between the two readings is the reflected power induced by the load impedance mismatch and is minimized during the welding operation by impedance matching techniques.
It has been found that by coordinating forging pressure and ultrasonic welding power levels so as to begin the weld as a very small point in comparison to its eventual final size and then syncronously ramping the pressure and welding power in order to progressively expand the weld outwardly to full size, any problems heretofore experienced with slippage of the welding tip relative to the workpiece have been substantially reduced. The act of welding produces a "thermal mound" on the workpieces between the welding tip and the anvil. The thermal mound is a geometrically unstable condition since it tends to cause the tip or the anvil to "walk away" from the workpieces or it causes the workpieces to be driven away from the welding area. Either of these actions diffuses the welding power and prevents optimal application of the weld spot. It is therefore an advantage of the progressive growth practice that the tip and/or anvil have an opportunity at low power stages to create a forged indentation on the thermal mound which is more appropriate to the power level and therefore lessens the inclination of the tip or anvil to walk away or eject the workpiece.
More importantly, the welds produced using such progressive weld growth procedures have proven to be structurally superior to welds produced when the welding power and pressure are stepped or otherwise abruptly increased. Shearing and welding actions occur in the annulus surrounding either a central friction locked area or a previously welded central area. A spot weld can best be made to grow coherently if the contact area is progressively enlarged by increasing contact pressure and welding power of the transverse strokes.
Efforts to progressively expand spot welds have been attempted in the past. For example, U.S. Pat. No. 3,610,506 which issued to Peter T. Robinson on Oct. 5, 1971, describes a method for ultrasonically welding using a varying welding pressure. A welding cycle is used by Robinson in which the vibratory energy is applied at the time the welding pressure is diminished although during the period in which vibratory energy is applied, the average value of the welding pressure with time continues to rise until termination of the welding cycle at which time a peak value is reached. At the conclusion of the welding cycle, the welding pressure drops to zero. The procedure followed by Robinson, which is intended for use in the semi-conductor industry where welding is effected at relatively low power levels in comparison to the power levels used in welding structural metal parts, allows the welding pressure to rise in a natural manner inherent to the pressure producing apparatus when contact is made to the workpiece and then full welding power is sharply applied. There is no coordination of the pressure and welding power or gradual ramping of both to effect a progressive gradual increase in weld size.
It has been found that by coordinating forging pressure and ultrasonic welding power, one is able to obtain superior results to those achieved by the method taught in the Robinson patent in that the welds produced thereby have proven to be structurally superior. In accordance with the welding practices discovered herein, the ultrasonic spot welds are started at low contact pressure and at low power levels which are then simultaneously increased resulting in an increase in weld size. The pressure and power are increased to a maximum. Welding power should always be insufficient to decouple the welding tip but sufficient to apply shear forces at the edge of any prevailing contact area. The present method has a sound phenomenological basis for controlled application with a variety of metals, thicknesses and contamination levels. The improved properties of the weld and reduced pre-cleaning costs give it strong potential for replacing resistance spot welding procedures. Further, it has been found that the method results in increased machine reliability. Abrupt power increases and decreases, heretofore commonplace in welding operations, tend to cause oscillatory motion or "ringing" which can be eliminated by more gradual increases and decreases in welding power applications. Step functions apply mechanical stresses to wire wound power resistors which have caused them to crack and fail. Step functions also lead to too vigorous actions at the beginning of the welds with the chance that decoupling can occur and large voltage spikes can be created in the piezo electric crystals which then pass into the power supplies and can cause transistor and fuse failures. Such chances of failure are virtually eliminated using progressive growth techniques.
Moreover, control systems heretofore used in prototype ultrasonic welding apparatus constitute little more than a collection of add-on circuits which include external power supplies. Typically, such systems failed to include a single source of synchronization signals which permit coordinated programming of pressure and power applications for the weld cycle. Such systems also experienced great difficulty in exactly repeating welding cycles. It is, of course, highly desirable to provide a single linear ramp signal which triggers both the power and pressure application simultaneously and which can concurrently control same. The system of the subject invention addresses the automation of the following functions of the welding cycle of the welding operation: initiation of the welding procedure with the movement of the anvil to engage the workpieces and the welding tip; application of a timed high pressure pulse between the anvil, the workpieces and the welding tip; return of the high pressure to a low pressure for the start of the weld; movement of the clamping system so as to close on the workpieces and press it against the clamping system; itiation of the power phase of the weld cycle after clamping pressure is attained; phase synchronous rise of both the anvil pressure and ultrasonic welding power; termination of the weld cycle; release of the clamping pressure and the release of the clamping system; release of the anvil pressure and return of the anvil to a predetermined level; and activation of the transport system to move sacrificial metal tape used for preventing or minimizing tip sticking over the welding tip a predetermined amount.
Commercially available welding machines fail to include such automatic sequencing systems. Available commercial welders generally have built into them little more than a weld operation initiation switch for closing the anvil upon the workpiece and welding tip. Upon contact of the anvil/workpiece/welding tip combination, a pressure switch is generally closed and preselected ultrasonic power and pressure levels are applied in a step function at a constant level for the time period of the weld. The constant power and pressure persist through a timed period. At the conclusion of the timed period, a switch opens and the ultrasonic welding power is removed. Simultaneously, the anvil is raised removing the clamping pressure from the workpiece. The workpiece is thereupon released permitting movement for the next welding cycle. As will be appreciated, such attempts are extremely basic and fail to offer the specific advantages contemplated by the present system.
Against the foregoing background, it is a primary object of the present invention to provide a progressive ultrasonic welding system.
It is another object of the present invention to provide such a system wherein the ultrasonic spot weld is initiated as a small point in comparison to the final size and then progressively expanded to full size.
It is yet another object of the present invention to provide such a system wherein the spot is started under low contact pressure and low power.
It is still another object of the present invention to provide such a system wherein the pressure and welding power are coherently and synchronously ramped to a predetermined maximum pressure and power to thereby progressively expand the weld to full size.
It is still yet another object of the present invention to provide such a system wherein the welding power applied is insufficient to decouple the welding tip but sufficient to apply shear forces at the edges of the prevailing contact area.
It is yet still another object of the present invention to provide such a system wherein the application of power and pressure is coordinated by the application of a linear ramp signal which triggers both power and pressure application.